This invention relates to the field of semiconductor circuit technologies. More particularly, some embodiments of this invention are directed to low-power slew rate detector circuits. In some embodiments, the slew rate detection circuits are used in class-D output power stages for edge rate control. However, the circuits can be used in any applications where accurate slew rate detection is desired.
In Class-D Audio amplifiers, the output signal is a Pulse Width Modulated (PWM) waveform, which drives the external speaker. Such Pulse Width Modulated Waveform looks very much like a square wave and therefore, it has significant high-frequency content that can disturb and interfere with other circuits using radio frequencies much higher than the PWM frequency. Below is shown a typical configuration of a first order Class-D stage and its input and output waveforms. FIG. 1 and FIG. 2.
A class-D amplifier, sometimes known as a switching amplifier, is an electronic amplifier in which all transistors operate as binary switches. They are either fully on or fully off. CLASS-D amplifiers employ rail-to-rail output switching, where, ideally, their output transistors virtually always carry either zero current or zero voltage. Thus, their power dissipation is minimal, and they provide high efficiency over a wide range of power levels. Their advantageous high efficiency has propelled their use in various audio applications, from cell phones to flat screen televisions and home theater receivers. Class-D audio power amplifiers are more efficient than class-AB audio power amplifiers. Because of their greater efficiency, class-D amplifiers require smaller power supplies and eliminate heat sinks, significantly reducing overall system costs, size, and weight.
Class D audio power amplifiers convert audio signals into high-frequency pulses that switch the output in accordance with the audio input signal. Some class D amplifiers use pulse width modulators (PWM) to generate a series of conditioning pulses that vary in width with the audio signal's amplitude. The varying-width pulses switch the power-output transistors at a fixed frequency. Other class D amplifiers may rely upon other types of pulse modulators. The following discussion will mainly refer to pulse width modulators, but those skilled in the art will recognize that class D amplifiers may be configured with other types of modulators.
FIG. 1 is a schematic diagram illustrating a class-D audio amplifier coupled to a slew rate circuit according to an embodiment of the present invention;
FIG. 1 shows a simplified schematic diagram illustrating a conventional class-D amplifier 100. The differential input audio signals INP and INM are input to comparators 101 and 102, where input signals INP and INM are compared with triangular waves VREF generated from an oscillator 103 to generate PWM signals 106 and 107. PWM signals 106 and 107 are coupled to the gates of transistors M1, M2, M3, and M4, respectively. Differential output signals OUTM and OUTP of the class D amplifier are respectively provided at terminals also labeled OUTM and OUTP. As shown in FIG. 1, output signals OUTM and OUTP are connected to a speaker load 110, which is represented by an inductor L1 and a resistor R1.
The traditional class D amplifiers have differential outputs (OUTP and OUTM) wherein each output is complementary and has a swing range from ground Vss to Vdd. The disadvantage of class-D amplification lies in the high frequency switching noise that is produced by the switching. This high frequency noise often resulted in EMI (Electronic-Magnetic Interference).
FIG. 2 is a waveform diagram illustrating the modulation of signals in the class-D amplifier of FIG. 1. As shown in FIG. 2, differential input signals, e.g., audio signals INM and INP, are compared with a triangular reference waveform VREF by two comparators as described above in connection with FIG. 1. The output signals of the comparators are pulse signals at a fixed frequency whose pulse width is proportional to the input signal. Two PWM signals are shown in FIG. 2 as OUTP and OUTM. The fast edges on the signals OUTP and OUTM can cause electromagnetic interference (EMI).
FIG. 3 is a diagram illustrating an example of EMI measurement of a class-D audio amplifier according to an embodiment of the present invention. The fast edges on the signals OUTP and OUTM can cause interference in the frequency range of 30 MHz to 1 GHz. FIG. 3 shows the result of a typical EMI test on a Class-D amplifier with a speaker load. The test is carried out in a test chamber, in which EMI signals are received at an antenna placed at a certain distance away from a device under test. As can be seen in FIG. 3, high frequency tones between 100 MHz and 600 MHz exceed the compliance mark 310. Therefore, there is a need to control the edges of the output signals, such that the high frequency content is reduced. Such control can be achieved, for example, by slowing down the gate control of M1, M2, M3, & M4 in FIG. 1. However, the actual slew rate of the output signals will eventually still depend on the PCB capacitance on the nodes OUTM & OUTP, the process, and temperature variation of the Class-D amplifier circuit. Therefore, there is a need for a more accurate detection of the real time edge rate in order to adjust for PCB capacitance on the nodes OUTM & OUTP, the process, and temperature variation of the Class-D amplifier circuit.
Conventional methods have been proposed for determining the slew rate of a signal, but they are not satisfactory. For example, in one approach, the timing of the input voltage at two reference levels is measured. The slew rate is then derived from the time difference. This approach requires an accurate time base circuit and high speed timer. In another approach, a switched capacitor circuit is used to determine the slew rate. This circuit requires switches and voltage references at the input, which is not suitable for use with high voltage input signals and the switches can create glitches on the input signal. In yet another approach, the slew rate is detected using a trans-conductance amplifier and the target application is for PWM supply control. Due to the trans-conductance amplifier the circuit is more complex and this added complexity and delay makes it slower, so it is not suitable for fast edge control.
Therefore, an improved slew rate detection circuit is highly desirable.